Currently, there are two international standards both specified in CCITT Recommendation G.711 for pulse code modulation. In the United States and Japan .mu.-law coding is used while in Europe and the rest of the world A-law coding is used. The two standards are highly incompatible, that is, if one standard is used to expand a bit stream that was originally compressed by the other, the result will be unintelligible. Accordingly, when a call is placed between countries using different PCM coding schemes, the network has to provide conversion through the use of a device that converts .mu.-law to A-law or vice versa. Unfortunately, such conversion may destroy the bit integrity of the signal that is required, for example, in 64 kbit/s ADPCM and in 64 kbit/s data transmission. Calls requiring preservation of bit integrity must therefore be routed via a "bit-transparent" sub-network. Mode switching between data (which requires bit-transparency) and PCM, which must be converted to the appropriate coding law when the call crosses boundaries, cannot be allowed.
Currently, however, network components such as bridges, terminals, switches, etc., cannot detect from an observation of the bit stream which type of coding is being employed. For a circuit to be used alternately for PCM voice and for data during a call would require that signaling be established between the terminals using the circuit so that the code conversion device may selectively be enabled or disabled. It would be a great improvement if a terminal could itself distinguish between the two types of PCM encoding or being used without losing bit integrity and without requiring signaling from the distant terminal.